Hydrogen storage alloy for use in alkaline storage batteries and method for production thereof

ABSTRACT

A hydrogen storage alloy, for use in alkaline storage batteries, having a CaCu 5 -type crystal structure and represented by the compositional formula MmNi x Co y Mn z M l-z  (wherein M represents at least one element selected from the group consisting of aluminum (Al) and copper (Cu); x is a nickel (Ni) stoichiometry and satisfies 3.0≦x≦5.2; y is a cobalt (Co) stoichiometry and satisfies 0≦y≦1.2; z is a manganese (Mn) stoichiometry and satisfies 0.1≦z≦0.9; and the sum of x, y and z satisfies 4.4≦x+y+z≦5.4). The hydrogen storage alloy includes a bulk region having a CaCu 5 -type crystal structure and a substantially uniform composition and a surface region surrounding said bulk region and having a graded composition. When the sum in percentage of numbers of cobalt (Co) atoms and copper (Cu) atoms present in the surface region is given by a and that in the bulk region by b, the relationship a/b≧1.3 is satisfied.

TECHNICAL FIELD

The present invention relates to a hydrogen storage alloy useful for anegative electrode of an alkaline storage battery and also to a methodfor production thereof.

BACKGROUND ART

A nickel-hydrogen storage battery has been recently noted as a newalkaline storage battery because of its high capacity, at least twice ashigh as that of nickel-cadmium batteries, and its environmental friendlynature. With the spread of portable instruments, this nickel-hydrogenstorage battery is expected to further increase its performance.

A hydrogen storage alloy, when incorporated in a negative electrode ofthe nickel-hydrogen storage battery, generally undergoes spontaneousoxidation to form an oxide layer on its surface. Accordingly, a hydrogenstorage alloy electrode fabricated from such a hydrogen storage alloy,when used as a negative electrode of the nickel-hydrogen storagebattery, presents a problem of low initial battery capacity that isattributed to the initial low activity of hydrogen storage alloy.

A method has been recently proposed, for example, in Japanese PatentLaying-Open No. Hei 5-225975, which immerses a hydrogen storage alloy inan acid solution, such as a hydrochloric acid solution, to remove anoxide layer formed on its surface.

This method contemplates to remove an oxide layer from a surface of thehydrogen storage alloy by immersing it in the acid solution. However,nickel and cobalt hardly elute into the acid solution so that activesites as of metallic nickel (Ni) and cobalt (Co) appear on the hydrogenstorage alloy surface.

Upon removal of the oxide layer by the above-described method, theactive sites as of metallic nickel and cobalt appear on the hydrogenstorage-alloy surface, so that the initial discharge capacity isincreased. The reduction of electrical contact resistance between alloyparticles also results to increase the high-rate discharge capacity to aslight degree. However, the electrical contact resistance between alloyparticles is still too high to achieve a marked improvement in high-ratedischarge capacity. Also, the method is still insufficient to preventthe buildup of pressure in the battery and improve a charge-dischargecycle life of the battery.

It is an object of the present invention to provide a hydrogen storagealloy which, when fabricated into an electrode for alkaline storagebatteries, can provide an excellent charge-discharge cycle lifeperformance, prevent the buildup of battery's internal pressure duringovercharge and improve high-rate discharge characteristics, and also toprovide a method for production thereof.

DISCLOSURE OF THE INVENTION

The hydrogen storage alloy of the present invention, for use in alkalinestorage batteries, has a CaCu₅-type crystal structure and is representedby the compositional formula MmNi_(x)Co_(y)Mn_(z)M_(l-z) (wherein Mrepresents at least one element selected from the group consisting ofaluminum (Al) and copper (Cu); x is a nickel (Ni) stoichiometry andsatisfies 3.0≦x≦5.2; y is a cobalt (Co) stoichiometry and satisfies0≦y≦1.2; z is a manganese (Mn) stoichiometry and satisfies 0.1≦z≦0.9;and the sum of x, y and z satisfies 4.4≦x+y+z≦5.4). Characteristically,the hydrogen storage alloy has a bulk region surrounded by a surfaceregion. The bulk region has a CaCu₅-type crystal structure and asubstantially uniform composition while the surface region has a gradedcomposition. When the sum of percentages by number of cobalt (Co) atomsand copper (Cu) atoms present in the surface region is given by a andthe sum of percentages by number of cobalt (Co) atoms and copper (Cu)atoms present in the bulk region is given by b, the relationship a/b≧1.3is satisfied.

In the present invention, the percentage by number of cobalt (Co) orcopper (Cu) atoms present in the bulk or surface region may be referredto in terms of atomic %.

As stated above, in the hydrogen storage alloy of the present invention,the bulk region is a region that has a CaCu₅-type crystal structure anda substantially uniform composition. The surface region is a region thatsurrounds the bulk region and has a graded composition. This surfaceregion is the region of the hydrogen storage alloy particle thatundergoes a change in composition when it is immersed in an acidtreating solution according to the production method of the presentinvention which will be described later. By this immersion treatment,any oxides present on an alloy particle surface is removed while cobaltand copper are reductively deposited. As a result, the surface region isallowed to contain the increased amounts of cobalt and copper atomscompared to the bulk region. As described above, when the sum ofpercentages by number of cobalt atoms and copper atoms present in thesurface region is given by a and the sum of percentages by number ofcobalt atoms and copper atoms present in the bulk region is given by b,the relationship a/b≧1.3 is satisfied. If a/b falls below 1.3, it maybecome difficult to obtain the effect of the present invention thatimproves a charge-discharge cycle life performance by lowering thecontact resistance between hydrogen storage alloy particles to therebyincrease a discharge capacity. It may also become hard to obtain thefurther effect of the present invention that not only prevents thebuild-up of battery's internal pressure during overcharge but alsoimproves high-rate discharge characteristics.

In the present invention, the region that encompasses a surface and itsvicinity and has a graded composition is defined as the surface region,as contrary to the bulk region that has a substantially uniformcomposition. The surface region is generally observed to have acomposition gradient such that the percentages by number of cobalt andcopper atoms present therein increase toward the surface. Accordingly,the sum of percentages by number of those atoms present in the surfaceregion, a, is determined by an average value in the surface region.Since the sum of percentages by number of those atoms measured at anintermediate depth of the surface region generally comes close to theaverage value in the surface region, the measured value may be taken asthe sum of percentages by number of cobalt and copper atoms present inthe surface region.

When the hydrogen storage alloy having the above-specified crystalstructure and compositional formula is used for negative electrodematerial of an alkaline storage battery, the negative electrode showsthe suppressed corrosion in an electrolyte to absorb the increasedamount of hydrogen. This is the reason why the present invention usesthe hydrogen storage alloy having such crystal structure andcomposition.

The surface region generally extends inwardly from an alloy particlesurface to the depth of 80 nm.

The hydrogen storage alloy electrode of the present invention, for usein alkaline storage batteries, is obtained by loading the hydrogenstorage alloy of the present invention in an electrically conductivesubstrate such as a punching metal.

The method for producing a hydrogen storage alloy of the presentinvention comprises a first step wherein alloy particles are preparedhaving a CaCu₅-type crystal structure and represented by thecompositional formula MmNi_(x)Co_(y)Mn_(z)M_(l-z) (wherein M representsat least one element selected from the group consisting of aluminum (Al)and copper (Cu); x is a nickel (Ni) stoichiometry and satisfies3.0≦x≦5.2; y is a cobalt (Co) stoichiometry and satisfies 0≦y≦1.2; z isa manganese (Mn) stoichiometry and satisfies 0.1≦z≦0.9; and the sum ofx, y and z satisfies 4.4≦x+y+z≦5.4), and a second step wherein the alloyparticles are immersed in an acid treating solution containing a cobaltcompound and a copper compound, each in the amount of 0.1-5.0% by weightbased on the weight of alloy particles, to remove oxide layers on alloyparticle surfaces and deposit cobalt and copper reductively so thatsurface regions are formed at alloy particle surfaces.

In the first step, alloy particles are prepared having the crystalstructure and compositional formula as specified above. Those alloyparticles are generally prepared by adding specific types of metals to aMisch metal consisting of a mixture of rare-earth metals. The techniqueused to prepare such alloy particles is not particularly specified. Theymay be prepared by casting the metal mixture into an ingot and thensubdividing the ingot, or alternatively, by utilizing a gas atomizing orroll quenching technique. In view of sinterability of resulting alloyparticles, the use of gas atomizing technique is preferred.

In the second step, the alloy particles are immersed in an acid treatingsolution containing a cobalt compound and a copper compound each in theamount of 0.1-5.0% by weight, based on the weight of alloy particles.This treatment not only removes oxide layers from alloy particlesurfaces but also allows reductive deposition of cobalt and copper,resulting in the formation of surface regions at the alloy particlesurfaces. Examples of acids useful for preparation of the acid treatingsolution include hydrochloric acid, nitric acid and phosphoric acid.

Examples of cobalt and copper compounds for addition to the acidtreating solution include cobalt chloride (CoCl₂), cobalt hydroxide(Co(OH)₂), copper chloride (CuCl₂) and copper hydroxide (Cu(OH)₂).

The cobalt and copper compounds are incorporated in the acid treatingsolution in concentrations of 0.1-5.0% by weight, respectively; for thereasons which follow. The respective loadings thereof, if exceed 5.0% byweight, cause cobalt and copper to be deposited in excessively largeamounts that result in the increased tendency of alloy particles to beoxidized and, if fall below 0.1% by weight, cause cobalt and copper tobe deposited in small amounts that result in the difficulty to satisfythe relationship a/b≧1.3. More preferably, the cobalt and coppercompounds are incorporated in the acid treating solution inconcentrations of 0.3-5.0% by weight, respectively.

Preferably, the acid treating solution is initially maintained at a pHin the range of 0.7-2.0. The pH of below 0.7 in some cases causes rapidoxidation of alloy particles that may dissolve even an interior of thehydrogen storage alloy. On the other hand, the pH of greater than 2.0may result in the insufficient removal of oxide layers.

The acid treating solution may further contain at least one organicadditive selected from the group consisting of 2,2′-bipyridyl,diethyldithio carbamate, 2-mercaptobenzothiazole and metanilic yellow.Such organic additives, if present, promote reductive deposition ofcobalt and copper. The organic additive may preferably be incorporatedin the amount of 5-50 ppm and contributes to further improvement ofbattery characteristics if kept within the specific range.

The method of producing a hydrogen storage alloy for use in alkalinestorage batteries, in accordance with the present invention, ischaracterized as including the step of loading the hydrogen storagealloy of the present invention in an electrically conductive substratesuch as a punching metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating hydrogen storage alloyparticles of the present invention.

FIG. 2 is a diagrammatic sectional view illustrating an alkaline storagebattery.

BEST MODE FOR CARRYING OUT THE INVENTION

Examples and Comparative Examples of the present invention are describedbelow in detail. However, the present invention is not limited to thefollowing Examples and can be practiced by adding suitable modificationswithin the range that does not depart from the gist of the presentinvention.

FIG. 1 is a schematic sectional view, illustrating hydrogen storagealloy particles of the present invention. The hydrogen storage alloyparticles 1 are configured such that each includes a bulk region 3 and asurface region 2 located toward a surface to surround the bulk region 3.Accordingly, the bulk region 3 is enclosed by the surface region 2. Asshown in FIG. 1, the depth of the surface region 2 is not necessarilyconsistent and may be varied from location to location. In general, thesurface region 2 extends from its surface mostly to the depth of 80 nm.

In the present invention, when the sum of percentages by number (atomic%) of cobalt atoms 4 and copper atoms 5 present in the surface region 2is given by a and the sum of percentages by number (atomic %) of cobaltatoms 4′ and copper atoms 5′ present in the bulk region 3 is given by b,the relationship a/b≧1.3 is satisfied.

The bulk region 3 is a region which has a CaCu₅-type crystal structureand a substantially uniform composition, while the surface region 2 is aregion which has been formed by the aforestated treatment with the acidsolution and has a composition different from that of the bulk region 3.The surface region 2 generally has a composition gradient such thatcobalt atoms 4 and copper atoms 5 are more concentrated toward asurface. Accordingly, the percentages by weight (atomic %) of cobaltatoms 4 and copper atoms 5 present in the surface region 2 is given byaverage values. Generally, the percentages by number of cobalt andcopper atoms present in the vicinity of an intermediate depth of thesurface region 2 determine those average values. Preferably, thepercentages by number of cobalt atoms 4 and copper atoms 5 aredetermined from measurements at several locations within the surfaceregion 2.

EXPERIMENT 1

In this Experiment 1, various hydrogen storage alloys, for use inalkaline storage batteries, were determined for the sum, a, ofpercentages by number (atomic %) of cobalt and copper atoms present inthe surface region, and the sum, b, of percentages by number (atomic %)of cobalt and copper atoms present in the bulk region located interiorof each hydrogen storage alloy particle. After calculation of a/b, itsrelation to battery characteristics was investigated.

Descriptions follow in the order of preparation of alloy particles,preparation of samples, assembly of alkaline storage batteries anddetailed analysis.

(Preparation of MmNi_(3.1)Co_(0.9)Mn_(0.6)M_(0.4) Alloy Particles)

Mm (Misch metal, a mixture of rare-earth metals, consisting, by weight,of 25% La, 50% Ce, 7% Pr and 18% Nd), and Ni, Co, Mn and Al (99.9% puremetal used for each), as starting materials, were mixed in a molar ratioof 1.0:3.1:0.9:0.6:0.4, allowed to melt in an electric arc furnace underargon atmosphere and then cooled naturally to produce an ingotrepresented by the compositional formulaMmNi_(3.1)Co_(0.9)Mn_(0.6)M_(0.4). This ingot was mechanicallysubdivided in the air in such a controlled fashion as to provide alloyparticles having an average particle size of 80 μm.

(Samples A1-A6 and Sample X)

A cobalt compound, cobalt chloride (CoCl₂), was added to an aqueoussolution of hydrochloric acid in the amount of 0.1% by weight, based onthe weight of alloy particles to be treated. A copper compound, copperchloride (CuCl₂), was also added to the aqueous solution of hydrochloricacid in the amount as indicated in Table 1, based on the weight of alloyparticles to be treated. The above procedures result in an acid treatingsolution having a pH of 1.0. The alloy particles were immersed in theabove-prepared acid treating solution maintained at 25° C., agitated for30 minutes, and then suction filtered. After filtration, the alloyparticles were washed with water and dried. As a result, hydrogenstorage alloy samples A1-A6 were obtained.

For comparative purposes, an aqueous solution of hydrochloric acidhaving a pH of 1.0, exclusive of cobalt chloride and copper chloride,was prepared. The alloy particles were immersed in this aqueous solutionof hydrochloric acid maintained at 25° C. and agitated for 30 minutes.After suction filtration, the alloy particles were washed with water anddried. As a result, an hydrogen storage alloy sample X was obtained.

(Assembly of Batteries)

100 parts by weight of each of the above-obtained hydrogen storage alloysamples and 20 parts by weight of a 5 wt. % solution of PEO(polyethylene oxide) in water, as a binder, were mixed to prepare apaste. This paste was applied (loaded) onto opposite sides of anelectrically conductive substrate comprised of a nickel-plated punchingmetal which was subsequently dried at room temperature and cut to aspecified size. As a result, hydrogen storage alloy electrodes suitablefor use in alkaline storage batteries were fabricated.

An AA-size positive limited alkaline storage battery (battery capacityof 1,000 mAh) was assembled by using each hydrogen storage alloyelectrode as a negative electrode. The alkaline storage battery alsoused a conventionally known sintered type nickel plate for a positiveelectrode, an alkali-resistant nonwoven fabric for a separator and a 30wt. % aqueous solution of potassium hydroxide for a liquid electrolyte.

FIG. 2 is a diagrammatic sectional view of an alkaline storage batteryconstruction as assembled. The alkaline storage battery includes apositive electrode 11, a negative electrode 12, a separator 13, apositive lead 14, a negative lead 15, a positive external terminal 16, anegative case 17 and a sealing cover 18.

The positive electrode 11 and negative electrode 12 are accommodatedwithin the negative case 17 in a spirally wound configuration with theseparator 13 between them. The positive lead 14 couples the positiveelectrode 11 to the sealing cover 18. The negative lead 15 couples thenegative electrode 12 to the negative case 17. An insulating gasket 20is provided to make a pressure-tight joint between the sealing cover 18and the negative case 17, so that the battery is closed. Placed betweenthe positive external terminal 16 and the sealing cover 18 is a coilspring 19 which is compressed responsive to the abnormal build-up of abattery's internal pressure to release a gas within the battery tooutside.

(Detailed Analysis)

The hydrogen storage alloy samples A1-A6 and the comparative hydrogenstorage alloy sample X were measured for percentages by number of atomsof elements present therein by using a scanning transmission electronmicroscope and an energy dispersive X-ray analyzer. The percentage bynumber of atoms of an element, as used herein, refers to a ratio of thenumber of atoms of each element to the total number of atoms of all theelements as detected by the scanning transmission electron microscopeand the energy dispersive X-ray analyzer, and is generally representedby the unit of atomic %.

A measurement sample was prepared by slicing an alloy particlerepresentative of each hydrogen storage alloy sample. A centrallylocated region of the measurement sample that had a CaCu₅-type crystalstructure and a substantially uniform composition was defined as thebulk region. The region surrounding the bulk region and having a gradedcomposition was defined as the surface region. The percentage by numberof atoms of each element present in the surface region was determined bythe value measured at the intermediate depth thereof, as stated earlier.

In the manner as described above, the percentages by number of cobaltand copper atoms present in the surface region and their sum, a, as wellas the percentages by number of cobalt and copper atoms present in thebulk region and their sum, b, were determined to calculate a value fora/b.

Specifically for the sample A1, the percentages by number of cobalt andcopper atoms present in the surface region and their sum, a, weredetermined as being 17.1 atomic %, 2.3 atomic % and 19.4 atomic %,respectively. On the other hand, the percentages by number of cobalt andcopper atoms present in the bulk region and their sum, b, weredetermined as being 14.9 atomic %, 0 atomic % (i.e., no copper atomdetected) and 14.9 atomic %, respectively. The value for a/b wasaccordingly 1.30.

(Evaluation of Performance Characteristics)

Each battery was measured for discharge capacity after 500 cycles. Eachbattery was charged at the 0.2 C rate at room temperature for 6 hoursand then discharged at the 0.2 C rate to 1.0 V. This unit cycle wasrepeated 500 times. The battery was charged once more to measure a 501stcycle discharge capacity (mAh) which was taken as the discharge capacityafter 500 cycles.

The following procedure was used to evaluate internal pressurecharacteristics of batteries: Each battery was charged at the 1.0 C ratewhile monitoring its internal pressure. When the internal pressurereached 10 kgf/cm², the time (min) was recorded as being indicative ofits internal pressure characteristics.

The following procedure was used to measure a high-rate dischargecapacity: After activation, each battery was charged at the 0.2 C rateat room temperature for 6 hours and then discharged at the 6.0 C rate to1.0 V to measure a capacity (mAh). The measured capacity (mAh) wasrecorded as the high-rate discharge capacity.

The measurement results, i.e., the ratio a/b, discharge capacity after500 cycles, internal pressure characteristics and high-rate dischargecharacteristics, for each of the batteries incorporating the samplesA1-A6 and comparative sample X, are given in Table 1.

TABLE 1 CoCl₂, CuCl₂, Capacity Internal High-rate Parts Parts (mAh)pressure discharge Sample by by Ratio after character- capacity No.weight weight a/b 500 cycles istics (min) (mAh) A1 0.1 0.1 1.30 760 130805 A2 0.1 0.5 1.32 765 135 805 A3 0.1 1.0 1.34 770 135 805 A4 0.1 3.01.35 770 135 805 A5 0.1 5.0 1.36 775 135 805 A6 0.1 7.0 1.36 740 120 790X 0 0 1.28 720 110 770

For the samples A1-A6 in accordance with the present invention,respectively prepared via treatment with the aqueous solution ofhydrochloric acid containing the cobalt compound, CoCl₂, and the coppercompound, CuCl₂, the ratio a/b of the sum, a, of percentages by numberof cobalt and copper atoms present in the surface region to the sum, b,of percentages by number of cobalt and copper atoms present in the bulkregion was found to satisfy the relationship a/b≧1.3.

The comparative sample X gave a value of 1.28 for a/b.

The batteries incorporating the samples A1-A6 that satisfied therelationship a/b≧1.3 gave higher values for discharge capacity after 500cycles and high-rate discharge capacity compared to the comparativebattery incorporating the comparative sample X. They also exhibitedsuperior internal pressure characteristics.

Next, the amount of cobalt chloride (CoCl₂) added to the acid treatingsolution was varied, by weight, to 0.5%, 1.0%, 3.0%, 5.0% and 7.0%,based on the weight of alloy particles to be treated, to investigate theeffect of cobalt chloride loading on battery performancecharacteristics.

As indicated in Table 2, sample groups B1-B6, C1-C6, D1-D6, E1-E6 andF1-F6 correspond to the cases where cobalt chloride was added in theamount by weight of 0.5%, 1.0%, 3.0%, 5.0% and 7.0%, respectively.

The measurement results, i.e., compositional ratio a/b for each sample,and discharge capacity after 500 cycles, internal pressurecharacteristics and high-rate discharge capacity for each battery, aregiven in Table 2.

TABLE 2 CoCl₂, CuCl₂, Capacity Internal High-rate Parts Parts (mAh)pressure discharge Sample by by Ratio after character- capacity No.weight weight a/b 500 cycles istics (min) (mAh) B1 0.5 0.1 1.33 775 130810 B2 0.5 0.5 1.34 780 135 810 B3 0.5 1.0 1.35 785 135 815 B4 0.5 3.01.36 795 135 820 B5 0.5 5.0 1.37 795 135 820 B6 0.5 7.0 1.37 745 120 795C1 1.0 0.1 1.35 795 130 810 C2 1.0 0.5 1.36 795 135 815 C3 1.0 1.0 1.37800 135 820 C4 1.0 3.0 1.38 805 140 820 C5 1.0 5.0 1.39 805 135 820 C61.0 7.0 1.39 755 120 795 D1 3.0 0.1 1.36 800 130 810 D2 3.0 0.5 1.36 805135 820 D3 3.0 1.0 1.38 810 135 825 D4 3.0 3.0 1.40 820 140 820 D5 3.05.0 1.41 820 145 820 D6 3.0 7.0 1.41 755 125 800 E1 5.0 0.1 1.39 805 130810 E2 5.0 0.5 1.40 805 130 820 E3 5.0 1.0 1.41 810 135 820 E4 5.0 3.01.42 815 145 820 E5 5.0 5.0 1.43 810 135 810 E6 5.0 7.0 1.43 750 125 790F1 7.0 0.1 1.39 755 130 800 F2 7.0 0.5 1.40 750 125 795 F3 7.0 1.0 1.41750 125 795 F4 7.0 3.0 1.41 745 125 795 F5 7.0 5.0 1.41 740 120 795 F67.0 7.0 1.41 735 115 790

As apparent from the results shown in Table 2, the batteriesincorporating samples containing 0.1-5.0% by weight of copper chloride,i.e., samples B1-B5, C1-C5, D1-D5, E1-E5 and F1-F5, show high values fordischarge capacity after 500 cycles, 740 mAh and greater. They also showhigh values for high-rate discharge capacity, 795 mAh and greater. It isthus understood that the cobalt chloride loading is preferably in therange of 0.1-5.0% by weight.

Based also on the results shown in Table 1, it is understood that thepreferred loadings of cobalt and copper compounds are both in the rangeof 0.1-5.0% by weight.

Although cobalt chloride and copper chloride were used in the Experiment1 for the cobalt and copper compounds, the similar results are obtainedwith the use of other types of cobalt and copper compounds, such ascobalt hydroxide (Co(OH)₂) and copper hydroxide (Cu(OH)₂).

Although an aqueous solution of hydrochloric acid was used as the acidtreating solution in the producing step of hydrogen storage alloy, i.e.,in step 2 of Experiment 1, the similar trend is observed with the use ofnitric acid or phosphoric acid.

EXPERIMENT 2

In this Experiment 2, the effect of a pH of the acid treating solutionon battery performances was investigated by varying the amount ofhydrochloric acid added to the acid treating solution in step 2. Here,cobalt chloride was used for the cobalt compound and copper chloride forthe copper compound.

First, an aqueous solution of hydrochloric acid was prepared whichcontained cobalt chloride and copper chloride in the amounts indicatedin Table 2, respectively based on the weight of alloy particles to betreated, and which was maintained at a pH in the range of 0.3-2.5 bycontrolled addition of hydrochloric acid. The alloy particles producedin the above Experiment 1 were added to the above-prepared aqueoushydrochloric acid solution, stirred for 30 minutes, suction filtered,washed with water and then dried. Each type of chloride compound wasadded to the samples G1-G5 in the amount of 0.5% by weight, to thesamples H1-H5 in the amount of 1.0% by weight, to the samples J1-J5 inthe amount of 3.0% by weight, and to the samples K1-K5 in the amount of5.0% by weight. The procedure of Experiment 1 was repeated using thesesamples to assemble batteries. The measurement results of dischargecapacity after 500 cycles, internal pressure characteristics andhigh-rate discharge capacity for each battery are given in Table 3.

TABLE 3 CoCl₂, CuCl₂, Capacity Internal High-rate Sam- Parts Parts ph of(mAh) pressure discharge ple by by Treating after character- capacityNo. weight weight solution 500 cycles istics (min) (mAh) G1 0.5 0.5 0.3730 115 775 G2 0.5 0.5 0.7 780 135 805 G3 0.5 0.5 1.0 780 135 810 G4 0.50.5 2.0 775 135 805 G5 0.5 0.5 2.5 740 135 780 H1 1.0 1.0 0.3 750 120780 H2 1.0 1.0 0.7 800 120 820 H3 1.0 1.0 1.0 800 135 820 H4 1.0 1.0 2.0800 135 820 H5 1.0 1.0 2.5 750 130 780 J1 3.0 3.0 0.3 750 120 780 J2 3.03.0 0.7 820 120 820 J3 3.0 3.0 1.0 820 140 820 J4 3.0 3.0 2.0 815 140820 J5 3.0 3.0 2.5 750 140 775 K1 5.0 5.0 0.3 750 120 775 K2 5.0 5.0 0.7810 120 810 K3 5.0 5.0 1.0 810 135 810 K4 5.0 5.0 2.0 810 130 810 K5 5.05.0 2.5 745 120 775

As apparent from the results shown in Table 3, the batteriesincorporating samples treated with the acid treating solution having apH kept within the range of 0.7-2.0, i.e., samples G2-G4, H1-H4, J2-J4and K1-K4, show high values for discharge capacity after 500 cycles, 780mAh and greater. They also show high values for high-rate dischargecapacity, 805 mAh and greater. These demonstrate that the pH of the acidsolution is preferably kept within the range of 0.7-2.0.

Although an aqueous solution of hydrochloric acid was used as the acidtreating solution in step 2 of Experiment 2, the similar trend isobserved with the use of nitric acid or phosphoric acid.

Although cobalt chloride and cobalt hydroxide were used in Experiment 2for the cobalt and copper compounds, the similar results are obtainedwith the use of other types of cobalt and copper compound, such ascobalt hydroxide and copper hydroxide.

EXPERIMENT 3

In this Experiment 3, the effect of inclusion of an organic additive inthe acid treating solution on battery performance characteristics wasinvestigated by varying the amount of 2,2′-bipyridyl, as a representingorganic additive added to the acid treating solution in step 2. Here,cobalt chloride was used for the cobalt compound and copper chloride forthe copper compound.

First, an aqueous hydrochloric acid solution having a pH of 1.0 wasprepared which contained cobalt chloride and copper chloride each in theamount of 1.0% or 3.0% by weight and 2,2′-bipyridyl in the amountindicated in Table 4, respectively based on the weight of alloyparticles to be treated. The alloy particles produced in Experiment 1were immersed in the above-prepared aqueous hydrochloric acid solution,stirred for 30 minutes, suction filtered, washed with water and thendried. As a result, samples L1-L6 and M1-M6 were obtained. The procedureof Experiment 1 was repeated using these samples to assemble batteries.Each battery was measured for discharge capacity after 500 cycles,internal pressure characteristics and high-rate discharge capacity. Themeasurement results are given in Table 4.

TABLE 4 CoCl₂, CuCl₂, 2,2′- Capacity Internal High-rate Sam- Parts Partsbi- (mAh) pressure discharge ple by by pyridyl after character- capacityNo. weight weight (ppm) 500 cycles istics (min) (mAh) L1 1.0 1.0 0 800135 820 L2 1.0 1.0 1.0 805 135 820 L3 1.0 1.0 5.0 810 140 825 L4 1.0 1.010.0 820 140 830 L5 1.0 1.0 50.0 810 140 825 L6 1.0 1.0 100.0 800 135820 M1 3.0 3.0 0 820 140 820 M2 3.0 3.0 1.0 825 140 820 M3 3.0 3.0 5.0830 140 830 M4 3.0 3.0 10.0 840 145 840 M5 3.0 3.0 50.0 830 140 830 M63.0 3.0 100.0 820 140 820

As apparent from the results shown in Table 4, the use of samplestreated with the acid treating solution containing the organic additive,2,2′-bipyridyl, in concentrations of 1.0-50.0 ppm results in theimproved battery performance characteristics. The addition of organicadditive in the amount of 5.0-50.0 ppm is particularly preferred.

Although 2,2′-bipyridyl was used as a representative organic additive inExperiment 3, the similar results are obtained with the use of othertypes of organic additives, such as diethyldithio carbamate,2-mercaptobenzothiazole and metanilic yellow.

The alloy particles used in the above Experiments were those produced byallowing a mixture of constituent metals to melt in an electric arcfurnace under argon atmosphere and then cool into an ingot andmechanically subdividing the ingot. The similar results are obtainedwith the use of alloy particles produced according to a gas atomizing orroll quenching technique.

The present invention can sustain the activity of alloy particlesurfaces and improve the electrical conductivity between alloyparticles. An electrode fabricated from such alloy particles, when usedas a negative electrode of a nickel-hydrogen storage battery, canprovide an excellent charge-discharge cycle life performance, preventthe buildup of battery's internal pressure during overcharge and improvehigh-rate discharge characteristics.

What is claimed is:
 1. A hydrogen storage alloy, for use in alkalinestorage batteries, having a CaCu₅-type crystal structure and representedby the compositional formula MmNi_(x)Co_(y)Mn_(z)M_(l-z), wherein Mrepresents at least one element selected from the group consisting ofaluminum (Al) and copper (Cu); x is a nickel (Ni) stoichiometry andsatisfies 3.0≦x≦5.2; y is a cobalt (Co) stoichiometry and satisfies0≦y≦1.2; z is a manganese (Mn) stoichiometry and satisfies 0.1≦z≦0.9;and the sum of x, y and z satisfies 4.4≦x+y+z≦5.4; wherein said hydrogenstorage alloy includes a bulk region having a CaCu₅-type crystalstructure, and a surface region surrounding said bulk region and havinga graded composition; wherein said surface region is formed by immersingsaid hydrogen storage alloy in an acid treating solution containing acobalt compound, a copper compound and an organic additive so that saidsurface region receives additional copper deposited from said acidtreating solution as a result of said immersing in said acid treatingsolution; and wherein, when the sum of percentages by number of cobalt(Co) atoms and copper (Cu) atoms present in said surface region is givenby a and the sum of percentages by number of cobalt (Co) atoms andcopper (Cu) atoms present in said bulk region is given by b, therelationship a/b≧1.3 for said graded composition is satisfied.
 2. Thehydrogen storage alloy as recited in claim 1, wherein said surfaceregion extends from a surface to a thickness depth of 80 nm.
 3. Ahydrogen alloy electrode incorporating the hydrogen storage alloy asrecited in claim 1 in the form of being loaded in an electricallyconductive substrate.
 4. A hydrogen storage alloy, for use in alkalinestorage batteries, having a CaCu₅-type crystal structure and representedby the compositional formula MmNi_(x)Co_(y)Mn_(z)M_(l-z), wherein Mrepresents at least one element selected from the group consisting ofaluminum (Al) and copper (Cu); x is a nickel (Ni) stoichiometry andsatisfies 3.0≦x≦5.2; y is a cobalt (Co) stoichiometry and satisfies0≦y≦1.2; z is a manganese (Mn) stoichiometry and satisfies 0.1≦z≦0.9;and the sum of x, y and z satisfies 4.4≦x+y+z≦5.4; wherein said hydrogenstorage alloy comprises alloy particles that each include a bulk regionhaving a CaCu₅-type crystal structure, and a surface region surroundingsaid bulk region and having a graded composition; wherein said surfaceregion is formed by immersing said hydrogen storage alloy in an acidtreating solution containing a cobalt compound, a copper compound and anorganic additive so that said surface region receives additional copperdeposited from said acid treating solution as a result of said immersingin said acid treating solution, and said graded composition of saidsurface region comprises a varying content of said copper that increasestoward a surface of said surface region and decreases toward said bulkregion; and wherein, when the sum of percentages by number of cobalt(Co) atoms and copper (Cu) atoms present in said surface region is givenby a and the sum of percentages by number of cobalt (Co) atoms andcopper (Cu) atoms present in said bulk region is given by b, therelationship a/b≧1.3 is satisfied.